Hello
As I understand it you are trying to create your Mosaic Dataset inside a Feature Dataset? Sadly this is not allowed.
Feature Datasets are used to predominantly manage geodatabase behaviours (for example topology, parcel fabrics, Network datasets, etc). They are also used to apply a spatial reference to the feature classes that inhabit the feature dataset and some people use feature datasets to group similar types of feature class.
You can only create your Mosaic Dataset in the "root" of the geodatabase.
Many thanks ed
